WASHINGTON (CNN) -- Democrats in Congress are going on the offensive Wednesday to force a shift in Iraq policy, the day before President Bush is expected to announce a troop drawdown.

Gen. David Petraeus testifies before the Senate Foreign Relations Committee on Capitol Hill on Tuesday.

Gen. David Petraeus, the top U.S. commander in Iraq, told Congress this week he could see the number of troops declining by 30,000 -- which would reduce the number to "pre-surge" levels -- by July 2008.

Bush is to address the nation on his plans Thursday night.Iraq war critic Sen. Jack Reed, D-Rhode Island, will deliver the Democratic response.

Democrats were quick to say that the recommended pullout is not a change in Iraq policy, but merely an end to the so-called surge -- the 30,000 additional troops sent to Iraq that began in January. The strategy was meant to fight growing sectarian and insurgent violence to allow for political stability. Watch Democratic Sen. Barack Obama call for pullout »

"The reduction to a pre-surge level is not a change in course," said Senate Armed Services Chairman Carl Levin, D-Michigan. "It's something which will happen anyway, just because of the rotation of our troops and the limit of 15 months for that rotation. So it's going to happen in any event. So returning to the pre-surge level is not a change in policy."

The White House insisted that the surge has been successful.

"The commitment on the ground has to reflect what is taking place," said White House spokesman Tony Snow. "Keep in mind the surge was designed to deal with problems on the ground. It has been dealing successfully with those.

"And you simply can't ignore what has been accomplished in the surge unless all you want to do is a cowardly job of just counting people there. What matters to the American people is that our forces are enjoying real success."

In an open letter to Bush, Sen. Hillary Clinton told the president Wednesday that simply returning to pre-surge levels is "unacceptable" and "too little, too late."

"Do not portray an unavoidable reduction in U.S. troops to pre-surge levels that would occur anyway as a marker of success," said the New York Democrat.

Democratic leaders met on Tuesday and talked about kicking their message into high gear. They've also met with some Republicans in hopes of reaching a compromise that would get the necessary 60 votes to allow any potential legislation to proceed.

During Tuesday's Capitol Hill testimony, GOP senators Lisa Murkowski of Alaska and Richard Lugar of Indiana -- who have not voted with Democrats to end the war -- pressed Petraeus to offer a plan.

Another Republican senator, Minnesota's Norm Coleman, told Petraeus that "Americans want to see light at the end of the tunnel" when it comes to the Iraq war.

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"Can we get a longer-term vision? Can we get a longer-term plan?" Coleman pressed the general. "Can we say that, yes, we can be down to half our troops in three years?"

Democratic leaders were to meet Wednesday to talk about how things stand with their Republican counterparts. Levin is working on a "super secret" plan with Republican Sen. John Warner of Virginia, Democratic aides said.

"This comes down to a handful of Republican senators," said Sen. Dick Durbin, the Senate's No. 2 Democrat. "We have four on our side, we need six more. We're talking to some. We're negotiating with some. We're trying to work out common language and agreements with them. Each one of them has a little different take on the issue."

Levin said some Republicans remain uneasy about staying with an "open-ended commitment in Iraq."

There are enough votes among moderate senators, Levin said, to force a change of course.

"I feel there are a lot of Republicans who want change beyond just lowering to pre-surge levels," said Levin, adding that a remaining question is whether a "pre-surge drawdown is a goal, or a firm timetable."

Other Republicans, however, such as Jon Kyl of Arizona, said they believed Petraeus' testimony was useful to shore up GOP support for Bush's war strategy.

The Petraeus hearing has "clearly taken the air out of the liberal-left notion that we need to leave Iraq immediately or very, very soon," Kyl said.

"To me, the whole point of this could unify Americans. What he has said is: This surge is working. And, as a result, we can begin to bring some troops home. Implication? The more it works, the more troops we can bring home."

But Democratic leaders face the prospect that any compromise with Republicans might alienate the party's left wing.

Asked whether that's possible, Democratic Sen. Barack Obama of Illinois, a presidential candidate who must appeal to the party's left wing, said: "It appears clear to me that the president is not willing to compromise short of Congress forcing him to accept a shorter timetable, and absent of that we are essentially engaging in a bunch of symbolic action there.

"What we need is a clear message from the Congress that it is time for us to change course, and it's time for us to do that," he told CNN.

Obama will detail his Iraq policy proposal later Wednesday during an appearance at Ashford University in Clinton, Iowa.

Excerpts of his speech made available to CNN reveal that Obama will call on Bush to "immediately begin to pull out troops engaged in combat operations at a pace of one or two brigades every month, to be completed by the end of next year."E-mail to a friend